Genetic studies of the cyc-l gene in yeast along with protein analysis of its gene product iso-l-cytochrome c has led to the identification of UAA and UAG nonsense mutants, missense mutants, frameshift mutants and mutations of the AUG initiation codon. The nucleotide sequences of these mutational lesions were deduced from amino acid changes in altered but functional, iso-l-cytochromes c. In addition, enrichmentment techniques have been developed for both forward and reverse mutations which are based, respectively, on the defectiveness and functioning of iso-l-cytochrome c for lactate utilization. Thisiso-l-cytochrome c system and the defined cyc-l mutants are being employed in numerous genetic studies, including investigations of mutagenesis, recombination and fine-structure mapping, nonsense mutants and their suppressors, initiation mutants and mechanisms of initiation of translation and structure-function relationships.